Superconducting magnet system with pulse tube cooler

ABSTRACT

A superconducting magnet system with an operating temperature T 1 &lt;3K which is disposed in a first helium tank ( 4 ) of a cryostat ( 1 ), wherein a second helium tank ( 2 ) is provided which is connected to the first helium tank ( 4 ) and contains liquid helium at an operating temperature T 2 &gt;3K, wherein a cooling means is provided in the first helium tank ( 4 ) which generates an operating temperature T 1 &lt;3K in that first helium tank ( 4 ) is characterized in that the cooling means is the cold end ( 19 ) of a pulse tube cooler ( 11 ) whose warm end ( 10 ) is disposed outside of the cryostat ( 1 ). The inventive magnet system minimizes the helium consumption thereby providing continuous measuring operation.

This application claims Paris Convention priority of DE 10 2004 012 452.3 filed Mar. 13, 2004 the complete disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention concerns a superconducting magnet system which is disposed in a first helium tank of a cryostat having an operating temperature T₁<3K, wherein a second helium tank is disposed above and connected to the first helium tank and contains liquid helium at an operating temperature T₂>3K, wherein the first helium tank includes a cooling means which generates the operating temperature T₁<3K in the first helium tank, wherein the cooling means is the cold end of a pulse tube cooler whose warm end is disposed outside of the cryostat.

A magnet system of this type is known per se from U.S. Pat. No. 5,220,800.

Superconducting magnet systems of this type generally comprise a cryostat with two chambers. A superconducting magnet coil is disposed in the first chamber and the second chamber serves as a helium supply and is at atmospheric pressure or slight overpressure at a temperature of approximately 4.2K. The two chambers communicate with each other such that helium can flow from the upper into the lower chamber where it is cooled to a temperature of considerably less than 4.2K using a further cooling unit which projects into that first chamber. A radiation shield reduces the incident radiation energy and is surrounded by a tank filled with a cryogenic liquid which cools the radiation shield.

Additional cooling units are conventionally used to further cool the helium in the first chamber, which relax the helium to a low pressure using a needle valve and pump it out of the first chamber. Pumping the helium out of the first chamber is disadvantageous, since it is removed from the system and the second chamber, which communicates with the first chamber, is slowly emptied thereby necessitating replacement of the helium in the second chamber at regular intervals.

In the magnet system according to U.S. Pat. No. 5,220,800 and DE 36 33 313 A1, the further cooling unit which projects into the first chamber pumps liquid helium out of the first chamber to cause further cooling of the helium bath in the first chamber as a result of that expansion.

Disadvantageously, helium is thereby constantly consumed by the refrigerator requiring corresponding refilling of helium into the apparatus. This helium may be supplied in liquid form, thereby necessitating a corresponding storage capacity. Moreover, helium is not always available in the amounts needed. Another possibility is to return the helium which escapes from the apparatus through liquefaction which, however, requires considerable expense with regard to equipment. In any case, the helium must be refilled in conventional magnet systems which necessitates interruption of the measuring operation and thereby involves substantial expense. For this reason, it is desirable to reduce the helium consumption of a magnet arrangement of this type.

U.S. Pat. No. 6,196,005 B1 discloses a cryostat configuration having an upper and a lower helium tank. The lower helium tank is cooled by a pulse tube cooler which passes from above through the upper and into the lower helium tank such the cold head of the pulse tube cooler projects into the lower helium tank. The pulse tube cooler cools the magnet system with a minimal loss of helium. However, in view of the fact that the pulse tube cooler passes through the upper helium tank and into the lower helium tank, the upper helium tank must have a sufficient amount of space to accommodate the pulse tube cooler, which is consequently no longer available for the storage of helium. The helium tank must therefore be larger than would otherwise be necessary in view of the helium requirements alone.

It is therefore the underlying purpose of the invention to propose a superconducting magnet system which is not susceptible to disturbances, wherein the helium consumption is minimized with simple means thereby eliminating undesired interruption of the measuring operation due to frequent refilling of helium. The system should also exhibit a compact construction.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention in that the warm end of the pulse tube cooler is disposed below the first helium tank.

Pulse tube coolers effect expansion and compression of the working gas using a shock wave front in a pulse tube. The shock wave front is thereby controlled by a rotating valve. The pulse tube is connected to a regenerator which provides heat exchange between the working gas and the regenerator material. After compression of the working gas, the gas flows through the regenerator to relax in the expansion chamber. The gas which is thereby cooled, absorbs heat from the surroundings of the expansion chamber thereby cooling those surroundings. Since the rotating valve need not be disposed in the direct vicinity of the magnet system, the pulse tube cooler is a smoothly running, low-wear cooling means which avoids moving parts in the low-temperature region.

Since the second helium tank is disposed above the first helium tank, it thereby serves a hydrostatic function to keep the first helium tank at atmospheric pressure.

As in the conventional means, the first helium tank contains a cooling means to cool the helium located therein. In contrast to the conventional systems, the inventive magnet system does not discharge helium from the helium tank, since the cooling means is a pulse tube cooler. The pulse tube cooler has its own, closed cycle. For this reason, no helium escapes into the atmosphere nor is the helium heated which would require renewed liquefaction of the gas and large amounts of energy and significant equipment expense. The helium consumption is minimized through the inventive magnet system thereby permitting continuous measuring operation.

Moreover, in accordance with the inventive magnet system, the warm end of the pulse tube cooler is disposed below the first helium tank. The configuration thereby permits use of a shorter pulse tube to decrease the overall height of the apparatus.

The invention realizes an evaporation-free superconducting magnet system, wherein the helium in the first helium tank is cooled via a cooling means in the form of a pulse tube cooler which is independent of the helium in the helium tank. The helium in the helium tank is not consumed during operation of the pulse tube cooler, which causes less frequent or optimally no refilling of the helium tank during operation of the magnet system. The inventive system therefore provides continuous measuring operation without having to organize the supply and refilling of helium. Moreover, the second helium tank may be smaller than in conventional magnet systems due to the reduced helium consumption. This reduces the overall size of the apparatus.

In a preferred embodiment of the magnet system, the pulse tube cooler has several, preferably two, stages. The second stage of the pulse tube cooler projects directly into the first tank, wherein the temperature of the second stage during operation is T<3K, whereby the helium in the first tank is further cooled directly and without removing helium thereby completely avoiding consumption of liquid helium in the first tank. The helium located in the second tank is also cooled through residual heat conduction via the thermal barrier. Minimizing the heat transfer into the second tank and suitable selection of the insulation properties of the thermal barrier ensures that no helium is discharged from the second helium tank, which is at atmospheric pressure or slight overpressure.

In a particularly suitable design of the insulation properties of the barrier, the second helium tank is slightly under-cooled and can be maintained at atmospheric pressure or slight overpressure through introduction of a heater therein, without having helium escape from the second helium tank. In this design, no helium is removed from the first and second tank during operation, which avoids the need to refill the cooling agent.

In a particularly preferred embodiment of the invention, one stage of the pulse tube cooler upstream of the cold end is thermally conductingly connected to a radiation shield disposed in the cryostat. The radiation shield can be cooled by the pulse tube cooler stage connected thereto.

In a particularly preferred fashion, the radiation shield connected to the pulse tube cooler stage surrounds the helium tanks and the pulse tube cooler replaces a tank of liquid nitrogen in the cryostat. In this case, supply of liquid nitrogen to the arrangement can be omitted. Due to omission of the nitrogen tank, the arrangement may be more compact.

The pulse tube cooler preferably comprises a regenerator material substance which has a phase transition at a low temperature of around 4K or below, in particular a magnetic phase transition. The phase transition increases the specific heat of the regenerator material to permit heat exchange from the working gas to the regenerator material, even at very low temperatures (T<4K).

In particular, for regenerator materials having a magnetic phase transition, the regenerator material is advantageously magnetically shielded in the cryostat thereby preventing disturbance of the main field by the magnetic phase transition.

In a further embodiment, the pulse tube cooler additionally or exclusively contains helium as the regenerator material. Since helium has no magnetic phase transition, it has no disturbing effects in connection with magnetic applications and is relatively inexpensive compared to other conventional regenerator materials. DE 199 24 184 A1 has already disclosed the use of high-pressure helium as a regenerator material.

In a particularly preferred embodiment of the invention, the section of the pulse tube cooler comprising the regenerator is disposed at a location in the cryostat having a minimum magnetic field during operation, e.g. radially outside of the magnet coil, approximately in the region of its central plane. Interaction between the regenerator material and the main magnetic field is thereby minimized.

The cryostat and the pulse tube cooler are preferably designed and dimensioned such that no helium must be refilled into the cryostat during operation to increase the user friendliness of the magnet system and permit continuous operation thereof.

In a particularly advantageous embodiment, the magnet system comprises a main field magnet coil and an active shielding coil which is disposed coaxially thereto and radially outside of the main field magnet coil, wherein the axes of the two coils are disposed vertically, and the cold end of the pulse tube cooler is disposed between the main field magnet coil and the shielding coil. The cold end of the pulse tube cooler is then in a low magnetic field or in a zero magnetic field, thereby minimizing or preventing disturbance of the main magnetic field by the pulse tube cooler.

In a further advantageous embodiment of the invention, a heating device is provided in the second helium tank to heat the helium. This is advantageous in that the pressure of the helium located in the helium tank can be regulated. The undercooled helium in the first helium tank can thereby be maintained at atmospheric pressure to realize a stable operating state.

Moreover, a valve for filling-in helium is advantageously provided on the cryostat, which is connected to at least one helium tank and through which helium can be refilled if e.g. helium has escaped via an overpressure valve.

The magnet system is preferably part of a magnetic resonance apparatus, such as an NMR spectrometer, a nuclear magnetic resonance tomograph or an ICR mass spectrometer. These apparatuses particularly depend on a homogeneous, stable and undisturbed magnetic field in a volume under investigation such that they considerably profit from the advantages of the inventive magnet system.

Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below may be used individually or collectively in arbitrary combination. The embodiments shown and described are not to be considered as exhaustive enumeration but have exemplary character for describing the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of a magnet system with installed pulse tube cooler; and

FIG. 2 is a schematic illustration of a preferred embodiment of an inventive magnet system with installed pulse tube cooler.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a magnet system with a first helium tank 4 which is disposed in a cryostat 1 and which contains a main field magnet coil 3 for generating a highly homogeneous magnetic field. A second helium tank 2 is disposed above the first helium tank 4 and is separated from the first helium tank 4 via a thermal barrier 5. The second helium tank 2 contains liquid helium at atmospheric pressure having a temperature of more than 3K, preferably 4.2K. The two helium tanks communicate with each other such that helium can flow from the upper into the lower chamber, where the helium is cooled (further cooled) to a temperature of considerably less than 3K, preferably 1.8K, using a pulse tube cooler 11. In the embodiment of FIG. 1, the pulse tube cooler 11 passes through the second helium tank 2 such that the warm end 10 of the pulse tube cooler 11 is disposed outside of the cryostat 1 and the cold end 19 of the pulse tube cooler 11 projects into the first helium tank 4 thereby cooling the helium located in the first helium tank 4 to the desired temperature. The arrangement of the pulse tube cooler 11 permits cooling of the helium thereby preventing helium from escaping from the helium tanks 2, 4 such that refilling of the helium tanks 2, 4 is not necessary during normal operation, thereby avoiding the demanding liquefaction of helium gas. For safety reasons, the magnet system may be provided with an overpressure valve through which helium can escape into the atmosphere in case of heating of the helium e.g. as a result of a quench of the main field magnet coil 3. In this case, refilling of helium into the second helium tank 2 may be required. Towards this end, the inventive magnet system comprises a fill-in valve 12.

The pulse tube cooler 11 which is integrated in the inventive magnet system has two stages to cool the helium below its boiling temperature at atmospheric pressure. The cold end 19 of the second stage 14 of the pulse tube cooler 11 projects into the first helium tank 4 to cool the helium in the first helium tank 4. The second helium tank 2 comprises a heating device to control the pressure in the helium tank e.g. to maintain the undercooled helium in the first helium tank 4 at atmospheric pressure. The first stage 13 of the pulse tube cooler 11 may be thermally conductingly connected to a radiation shield 15 located in the cryostat 1. The radiation shield 15 reduces incoming radiation energy. The radiation shield 15 may be cooled via the pulse tube cooler 11 through thermal connection to the radiation shield 15 such that a separate nitrogen tank 16 can be omitted. For this reason and through reduction of the size of the second helium tank 2, the inventive magnet system can be realized with compact size compared to conventional magnet systems.

The pulse tube cooler 11 is preferably disposed within a vacuum safety device which projects through the radiation shield 15 and the second helium tank 2 and is mounted to the vacuum safety device in a pressure-tight manner. The vacuum safety device comprises side walls 8 of a material with poor conducting properties, e.g. stainless steel, and an end piece 9 which contacts the cold end 19 of the pulse tube cooler 11 and is made from a material having good conducting properties, e.g. copper, such that heat exchange between the liquid helium in the first helium tank 4 and the pulse tube cooler 11 is effected mainly via the cold end 9 of the pulse tube cooler 11.

The pulse tube cooler 11 comprises a regenerator material having a phase transition in order to generate the required low temperatures. The phase transition increases the volumetric specific heat of the regenerator material and permits cooling of the helium to less than 3K. Pb and rare earth compounds such as e.g. HoCo, Er₃Ni, ErNi, GdAlO₃ and ErNi_(0.9)Co_(0.1) are suitable regenerator materials. These materials, however, have a magnetic phase transition which may be disturbing in connection with magnetic applications. The inventive magnet system therefore provides magnetic shielding of the regenerator material in the cryostat 1 via e.g. a μ-metal foil which surrounds the pulse tube cooler 11 or using a highly-conducting housing to shield the fluctuating magnetization. A superconducting housing can also surround the pulse tube cooler 11 to minimize the influence of the above-mentioned disturbing effects resulting from the magnetic phase transition of the regenerator material.

FIG. 2 shows a particularly advantageous embodiment of the inventive magnet system, wherein the warm end 10 of the pulse tube cooler 11 is disposed below the first helium tank 4. This arrangement permits use of a shorter pulse tube cooler 11, since the pulse tube cooler need not pass through the second helium tank 2. In addition to the main field magnet coil 3, the inventive magnet system may comprise an active shielding coil which is disposed outside of and coaxial to the main field magnet coil 3 and which shields the main magnetic field towards the outside. The pulse tube cooler 11 may be disposed such that the cold end 19 of the pulse tube cooler 11 is disposed between the main field magnet coil 3 and the shielding coil. Due to the shielding function of the shielding coil, the magnetic field between the main field magnet coil 3 and the shielding coil is zero or nearly zero. This arrangement of the pulse tube cooler 11 minimizes interaction between the regenerator material and the main magnetic field even if regenerator materials having magnetic phase transitions are used. The inventive magnet system therefore permits use of conventional pulse tube coolers without having to accept the usually associated disadvantages.

The inventive magnet system improves the measuring operation since the number of helium refilling operations can be considerably reduced. The second helium tank 2 which, in conventional magnet systems, contains a relatively large supply of helium in order to be able to supply helium to the first helium tank 4 over a longer time period, may be considerably smaller in the inventive magnet system. The second helium tank 2 thereby mainly serves a hydrostatic function, i.e. maintains the atmospheric pressure in the helium tanks 2, 4. In contrast to the conventional magnet system, the inventive magnet system uses no helium from the helium tanks 2, 4 for the cooling process. The inventive design is therefore suited for dry systems and thereby offers a broader application spectrum.

In total, a compact magnet system is obtained which is easy to handle and which avoids heating of the helium in the helium tank and consequently also re-liquefaction, to permit continuous measuring operation and largely spares the staff the inconvenience of having to provide and refill helium.

The magnet systems of FIGS. 1 and 2 are part of a high-resolution NMR apparatus at a high magnetic field of around or over 20 Tesla.

LIST OF REFERENCE NUMERALS

-   1 cryostat -   2 second helium tank -   3 main field magnet coil -   4 first helium tank -   5 thermal barrier -   8 side walls of the vacuum safety device -   9 end piece of the vacuum safety device -   10 warm end of the pulse tube cooler -   11 pulse tube cooler -   12 fill-in valve -   13 first stage of the pulse tube cooler -   14 second stage of the pulse tube cooler -   15 radiation shield -   19 cold end of the pulse tube cooler 

1. A magnet system having a superconducting magnet disposed in a cryostat, the system comprising: a first helium tank disposed in the cryostat, the superconducting magnet being disposed in said first helium tank, said first helium tank containing liquid helium at an operating temperature T₁<3K; a second helium tank disposed in the cryostat above and in liquid helium communication with said first helium tank, said second helium tank containing liquid helium at an operating temperature T₂>3K; and a pulse tube cooler, said pulse tube cooler having a cold end disposed in said first helium tank to generate said operating temperature T₁<3K, said pulse tube cooler having a warm end disposed outside of the cryostat, below said first helium tank.
 2. The magnet system of claim 1, wherein the magnet system is structured and dimensioned for magnetic resonance measurements.
 3. The magnet system of claim 1, wherein said pulse tube cooler has several stages.
 4. The magnet system of claim 1, wherein said pulse tube cooler has two stages.
 5. The magnet system of claim 3, wherein a stage of said pulse tube cooler upstream of said cold end is thermally conductingly connected to a radiation shield disposed in the cryostat.
 6. The magnet system of claim 5, wherein said radiation surrounds said first and said second helium tanks to replace a liquid nitrogen tank in the cryostat.
 7. The magnet system of claim 1, further comprising a thermal barrier disposed in the cryostat between said first and said second helium tanks, said thermal barrier having a residual conductivity to further cool helium in said second helium tank and minimize consumption of liquid helium therein.
 8. The magnet system of claim 1, wherein said second helium tank has a heater to adjust a pressure in said second helium tank through heating.
 9. The magnet system of claim 1, wherein said pulse tube cooler contains a regenerator material substance having a phase transition.
 10. The magnet system of claim 9, wherein said phase transition is a magnetic phase transition.
 11. The magnet system of claim 10, wherein said the regenerator material is magnetically shielded in the cryostat.
 12. The magnet system of claim 1, wherein said pulse tube cooler contains helium as a regenerator material.
 13. The magnet system of claim 1, wherein a section of said pulse tube cooler which contains a regenerator is disposed at a location in the cryostat having a minimum magnetic field during operation.
 14. The magnet system of claim 1, wherein the cryostat and said pulse tube cooler are designed and dimensioned such that helium must not be refilled into the cryostat during operation of the magnet system.
 15. The magnet system of claim 1, wherein the magnet system comprises a main field magnet coil and an active shielding coil which is disposed coaxially with respect to and radially outside of said main field magnet coil, wherein a common axis of said main field and said active shielding coils is vertical and said cold end of said pulse tube cooler is disposed between said main field magnet coil and said shielding coil.
 16. The magnet system of claim 1, wherein said cryostat has a valve for filling-in helium, said valve being connected to at least one of said first and said second helium tanks. 